The number of youth living with HIV continues to rise, and they are disproportionately represented at each stage of the care continuum. Most relevant to this application, it is estimated that less than half of HIV-infected youth in the United States (US) have been diagnosed with HIV, and AIDS-related deaths among youth have increased over the past decade despite decreased death rates among all other age groups, pointing to the urgent need for increased testing among youth. Black and Latino youth are at increased risk of poor HIV-related outcomes and have disparate testing rates as compared to White youth. Mobile Health (mHealth) technology is a powerful and relevant tool which represents a promising approach for improving outcomes among youth living with HIV. Youth are avid adopters and heavy users of smartphones and digital technologies, and these technologies offer opportunities to tailor interventions to developmental stages and personal needs. Importantly, these technologies are capable of delivering interventions in real-time and in ecologic settings. This creates an opportunity to remotely reach youth through mobile and connected health approaches to strengthen their HIV care continuum engagement and treatment outcomes. In response, our study team has developed the mLab App, an innovative mobile and connected technology that combines HIV prevention information with push notifications/reminders to complete HIV testing and an automated image processing feature to provide accessible, objective, secure, and real-time feedback on home-based OraQuick (lateral flow assay) HIV test results. The mLab App also contains an innovative automated data collection and a results reporting feature. Findings from our preliminary work in New York City indicate that youth perceive the mLab App as useful, easy to use, and effective at improving health outcomes and intend to use the technology. Findings from our preliminary work in Africa support the sensitivity and specificity of the imaging algorithm for interpreting lateral flow assay results. Theoretically-guided by the Health Information Technology Usability Evaluation Model (Health-ITUEM), the proposed project will refine and test a next- generation diagnostic intervention delivered on a mobile platform to improve HIV testing and linkage-to-care outcomes among youth living with and at-risk for HIV. Well in line with the objectives of RFA-MH-18-606, we will conduct a careful, iterative process of technology refinement based on input from end users, experts, and our youth advisory board. We will then enroll 500 high-risk youth (age 17-29 years) in a 12-month RCT to assess differences in HIV testing rates and linkage to care between the intervention (mLab App) and control arm. Finally, we will analyze paradata, defined as auxiliary data that capture details about the process of interaction with the technology, to understand the effect of user engagement of the mLab App on improving HIV testing rates and linkage to care. Interventions delivered through mHealth technology represent a promising approach for improving outcomes among youth. Given the pervasiveness, low cost, and convenience of mobile technology, the mLab App holds promise to help achieve the goals of the National HIV/AIDS Strategy in the US by increasing the number of persons living with HIV who know their serostatus, decreasing HIV-related disparities, and ultimately reducing the risk of HIV transmission and acquisition.